Signal modulators are utilized in a number of applications, for instance as part of the conversion from analogue signals to digital signals or vice versa. Time-encoding modulators are modulators that encode input data into a time-encoded data stream.
One particular form of time-encoding is pulse-width modulation (PWM). In a PWM signal an input value is encoded by the duration of a given output signal level, e.g. the duration or width of pulse of a first signal level, compared to the duration of any period(s) of any other signal level(s) in a cycle period. For a conventional two-level PWM signal, the input signal value may be encoded by the duty cycle of a pulse of a first signal level within the cycle period, i.e. the proportion of the cycle period spent at the first output signal level.
One particular application of a time-encoding modulator is as part of an analogue-to-digital converter (ADC) having a controlled oscillator, such as a voltage-controlled-oscillator (VCO).
In a VCO based ADC the input analogue signal may be used to control the VCO, which thus outputs an oscillation signal with a frequency dependent on the value of the input signal. The frequency of the oscillation signal is determined, for instance by counting the number of cycles of the oscillation signal in a defined frame period, to provide a digital output indicative of the value of the input signal. The VCO can be implemented by a relatively simple ring oscillator, which is relatively low power, and thus VCO based ADCs can be relatively efficient in terms of power requirements and circuit area compared to alternative ADC architectures such as continuous-time or discrete-time sigma-delta ADCs.
However VCOs, especially those based on ring oscillators, typically have relatively poor linearity. Thus if the VCO is controlled directly by the analogue input signal the resulting digital output signal may have poor linearity. Whilst this may be acceptable for some applications, the poor linearity is disadvantageous for audio applications.
A time-encoding modulator, for instance a PWM modulator, can be used to improve the linearity of a VCO based ADC by modulating the input analogue signal into a time-encoded signal which varies between two voltage states, where the value of the input signal is encoded by the relative durations of each state. The time-encoded signal is then used to control the VCO. As the VCO thus only operates with two input voltage levels the output is inherently linear. The average frequency of the oscillation signal in a frame period depends on the time spent at each voltage level, and thus depends on the value of the input signal.
Typically the time-encoding modulator used in such applications comprises an asynchronous sigma-delta modulator (ASDM). FIG. 1 illustrates one example of an ASDM 100, which in this example is a differential ASDM, but ASDMs for single-ended signals would have a similar general design as would be understood by one skilled in the art. The differential input signal at inputs INP and INN is combined with a feedback signal and supplied to integrator 101 which provides the integrated signal to hysteretic comparator 102. As will be well understood by one skilled in the art, this arrangement provides, at a first output OUTP, a PWM signal that varies between two voltage states, say a high level VH and a low level VL, where the relative duration of each state depends on the input signal. For the differential arrangement illustrated a second output OUTN is in antiphase with OUTP so as to provide a differential PWM signal.
The integrator 101 typically comprises an op-amp 103 with integrator capacitors 104. When used for audio ADC applications, the performance requirements typically mean that a relatively large op-amp 103 with a relatively high power consumption is implemented. The integrator capacitors also need to be relatively large. Generally there is a desire for circuitry to be as small as possible and, especially for battery powered devices, power consumption is a concern and low power operation is desirable.